linux-ext4 - Re: [LSF/FS TOPIC] Ext4 snapshots status update

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Date:	Wed, 30 Mar 2011 03:33:23 -0700
From:	Joel Becker <jlbec@...lplan.org>
To:	Amir Goldstein <amir73il@...il.com>
Cc:	Tao Ma <tm@....ma>, linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	Theodore Tso <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Josef Bacik <josef@...hat.com>
Subject: Re: [LSF/FS TOPIC] Ext4 snapshots status update

On Wed, Mar 30, 2011 at 08:05:38AM +0200, Amir Goldstein wrote:
> Just wanted to clarify there are 2 differences I notice between mmap
> write to a hole
> and mmap write to COWed file with ENOSPC:
> 
> 1. A "good" application can avoid mmap write to a hole.
> 
> 2. when initiating a hole, the mkwrite callback is in used (in ext4) to
> reserve disk space for delayed allocation when a page becomes writable.
> with COW a page may already be writable when the flush encounters COW
> with ENOSPC. that flush can even happen after the application has exited,
> so the data will be dropped on the floor silently (like in ext3).

	ocfs2 doesn't have delayed allocation yet, so we try and fail
the allocation in write_begin() right under mkwrite().

Joel

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			http://www.jlbec.org/
			jlbec@...lplan.org
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